Compositions of carbonaceous-type asteroidal cores in the early solar system

The parent cores of iron meteorites belong to the earliest accreted bodies in the solar system. These cores formed in two isotopically distinct reservoirs: noncarbonaceous (NC) type and carbonaceous (CC) type in the inner and outer solar system, respectively. We measured elemental compositions of CC-iron groups and used fractional crystallization modeling to reconstruct the bulk compositions and crystallization processes of their parent asteroidal cores. We found generally lower S and higher P in CC-iron cores than in NC-iron cores and higher HSE (highly siderophile element) abundances in some CC-iron cores than in NC-iron cores. We suggest that the different HSE abundances among the CC-iron cores are related to the spatial distribution of refractory metal nugget–bearing calcium aluminum–rich inclusions (CAIs) in the protoplanetary disk. CAIs may have been transported to the outer solar system and distributed heterogeneously within the first million years of solar system history.

In Eq. 1, Ci, CL, f, and DE represent the bulk composition of the liquid, the bulk composition of the remaining liquid, the crystallization step, and the partition coefficient of an element, respectively. The models in this study used constant 0.001 for each mass step. The concentration of an element in the solid (Cs) derived from each mass step is calculated using the bulk composition of the remaining liquid and the partition coefficient of the element in that step: The partition coefficient of an element is strongly influenced by the S and P contents of the liquid and varies at each small step. DE is parameterized using Eq. 3 (26).
D0 is the partition coefficient of an element in the S-and P-free system. β is a constant specific to an element related to S and P in the liquid. Fe domains represent the fraction of free Fe atoms available in the liquid (37). Fe domains in the Fe-Ni-S-P system were calculated by Eq. 4, and βS+P of an element in the Fe-Ni-S-P system was calculated using Eq. 5 (20).
')( = : XS and XP are the molar fractions of S and P in the liquid, respectively. βS and βP are the beta values for each element in the Fe-S and Fe-P systems, respectively. The scattered interelement trends of group IIIAB can be caused by the equilibrium mixing of solid and liquid (trapped melt) (31), which is called the trapped-melt model. A recently revised version of the trapped-melt model considers the formation of troilite in the trapped melt (32).
The relationship between the trapped melt (CTrapped melt) and the solid (CTrapped melt solid) that crystallized from the trapped melt can be expressed using Eq. 6: where x denotes the mass fraction of the trapped melt that solidifies to form troilite. In this study, we consider the formation of all groups and evaluate the fraction of trapped melt for each group.
We used the composition data of Cr, Co, Ni, Cu, Ga, Ge, As, Sb, Ru, Re, Os, W, Ir, and Au determined by NAA. Some Ru and Os data and all Rh, Pd, and Mo data are from ICP-MS data in the literature. Phosphorus concentrations are from modal analyses (70). Details of the data sources are shown in table 1. The models in this study are based on element vs. As trends. The use of As and Au as abscissa in the fractional-crystallization models was first used in group IIIAB (31). Arsenic and Au have almost the same geochemical behavior during fractional crystallization in metallic melts (37). These two elements have lower partition coefficients so that they have a larger range of concentrations. For an iron group, the range of Au and As concentrations is larger than their INAA analytical uncertainties (1.5 to 3% for Au, 4 to 6% for As) (22). As a result, element-Au and element-As diagrams provide better estimates of the position of meteorites in the fractional-crystallization tracks. Despite the almost identical behaviors of As and Au during fractional crystallization, the partition coefficient of Au is relatively poorly understood in low-S melts (32,37). Thus, element vs. As trends were used to evaluate our fractional-crystallization models.
The bulk compositions of the CC-type groups were determined by the trial-and-error method. The first solids obtained by the models are assumed to be the lowest-As irons or solids with similar compositions. We assume the lower boundary of the envelope of an element-As trend overlaps with the SFC solid track. The initial elemental concentrations are adjusted to fit the model tracks with as many element-As trends as possible at the same time. An optimal initial S content is thereby obtained. The adjacent sulfur contents bracketing the optimal S content are tested at increments of 0.5% (groups IVB and IID) or 1% (other groups), enabling us to take the analytical and modeling uncertainties into consideration in the models.
Comparison between our model and the HSE-based model for group IID A recent study used the same modeling method (based only on HSEs) for group IID and resulted in initial bulk concentration estimates of 10 wt % S and 1 wt P (23); this S content is drastically different from our S value of 0.5 ± 0.5 wt % and a prior estimate of 0.7 wt % (22). The bulk Re, Ir, and Pt concentrations in the low-S models (this study and (22)) are two to three times higher than those in the high-S model (23) (70). It cannot be excluded that the polished section was a nonrepresentative sample of Wallapai. However, for the large section, such a low troilite content for an evolved iron is more consistent with the low bulk S content in group IID. Another notable inconsistency in using 10 wt % S is that it cannot explain the Ga vs. As trend (fig. S1A); Wallapai would remain outside the trapped-melt-model envelope on the Ir vs. Au diagram ( fig. S1C). Due to the similarity of As and Au in fractionalcrystallization parametrization, the Ir vs. Au and Ir vs. As diagrams should show similar crystallization sequences and fractions of trapped melt in an optimal fractional-crystallization model (22,30,31).    Table S1. Mean compositions of irons in groups IIC, IID, IIF, IVB, and the South Byron Trio (SBT). The meteorites in each group are arranged in order of increasing As.